The first thing you will want to do before you get a headshot; long before you go looking for an agent; long before you pick up the industry newspapers....is to find a school, a coach or independent teacher to teach you the craft of acting. If you are already in a school that has a theater department or even if they just put on plays from time to time, do what you can to become a part of that. Schooling is something you will be doing throughout your career as an actor. Getting a proper education into the world of the actor is a great way for you to discover work, and probably most important of all, make important connections.
After some time as a student, the next things you'll need are headshots and a reel. These will be your business card or calling card. Without headshots especially, no one will want to see you. Agents, managers, casting directors, etc. will not want to see you...period. Get your headshots done and the world will open to you with great ease.
The acting profession is a unique blue-collar job. Just like any other job, you will have to apply for it. We do so by auditioning. It is a process no one likes on either side of the table, but it is the best we've got.
BOOKS TO READ:
An Actor Prepares by Konstantine Stanislavski - (A work of fiction, but the principles there are the basics of modern acting.)
An Actor Behaves by Tom Markus - (This book was originally published in 1992, but I think it has since been updated.)
On Acting by Sanford Meisner
These three are the basic bibles of the actor.
I love that you want to become an actress! A way that you can practice improving your skills include practicing by yourself within the mirror. You can also record yourself, show others, and gain feedback on that to improve!
If possible, you can also try to take acting classes.
If you don't have one, you can also look into different casting agencies and get yourself a manager.